1. Field of the Invention
The present invention relates to a nuclear magnetic resonance imaging, and more particularly, to an improvement of an image quality of the nuclear magnetic resonance images to be obtained and an operation efficiency in the nuclear magnetic resonance imaging.
2. Description of the Background Art
The nuclear magnetic resonance imaging (referred also as MRI hereinbelow) is a technique for imaging microscopic chemical or physical information of an imaging target object by utilizing tile so called nuclear magnetic resonance phenomenon according to which the nucleus having a characteristic magnetic moment placed in a homogeneous static magnetic field can resonantly absorb the energy of the high frequency magnetic field of a specific resonant frequency.
In an MRI apparatus for carrying out such a nuclear magnetic resonance imaging, depending on a type of an imaging pulse sequence to be used, numerous different manners of switching of the gradient magnetic fields to be superposed onto the static magnetic field will be required.
Examples of the conventionally known imaging pulse sequence to be used in the nuclear magnetic resonance imaging include the usual imaging sequence such as the spin echo sequence and the field echo sequence, the ultra high speed imaging sequence such as the echo planar sequence, and the nuclear magnetic resonance angiography sequence for obtaining the distribution or the speed of the blood flow in the blood vessels.
Each of these imaging pulse sequences is associated with a characteristic manner of switching of the gradient magnetic fields, and such a switching of the gradient magnetic fields is known to generate the eddy currents on the thermal shielding for the superconducting magnet as well as on the RF shielding, while generating the coupling currents on the shim coils. These transient currents can affect the temporal and spatial characteristics of the gradient magnetic fields to cause the serious degradation of the image quality in the nuclear magnetic resonance images to be obtained such as blurring.
In general, the level of the blurring caused by the transient magnetic fields produced by these transient currents differs at different spatial position, so that it can not be compensated by the usual image quality restration technique such as the inverse filtering at k-space (spatial frequency space).
Conventionally, this problem of the image quality degradation due to the transient currents caused by the switching of the gradient magnetic fields has been at least partially resolved as follows.
First, there has been a proposition for a method to compensate the time response of the eddy currents generated on the thermal shieldings for the superconducting magnet and the high frequency magnetic field shieldings by modulating the currents to be supplied to the gradient coils for generating the gradient magnetic fields by the components corresponding to the inverse response of the time response of the eddy currents.
However, even when the time response of the eddy currents is completely compensated by this method, there still remains the degradation of the image quality due to the fact that the transient magnetic fields produced by the transient currents have the spatial non-liearlity and the field center which are different from those of the desired gradient magnetic fields, which is particularly noticeable at spatial positions rather distanced from the field center. In order words, in this method, the residual eddy current magnetic field can be completely nullified only at a point at which the eddy current compensation has been made, and the significant amount of the residual eddy current magnetic field will be still present elsewhere, where the strength of the residual eddy current magnetic field at each spatial position increases in proportion to the difference of the relative strength of the residual eddy current magnetic field at each spatial point with respect to the residual eddy current magnetic field strength at the eddy current compensation point.
Moreover, this method does not account for the coupling currents generated on the shim coils at all, so that it can only be a partial resolution of the whole problem at best.
Furthermore, there has been no generally applicable analysis of the magnetic field distribution after the eddy current compensation available conventionally, so that the optimal eddy current compensation accounting for the spatial distribution of the residual eddy current magnetic field has been unavailable conventionally.
On the other hand, there has been a more advanced proposition aimed at the full resolution of this problem of the image quality degradation due to the transient currents including the eddy currents as well as the coupling currents, concerning the use of the so called active shield gradient coils (ASGC) comprising the primary gradient coil as inner coil and the shielding coil as outer coil enclosing the inner coil such that the desired gradient magnetic field can be generated only within a space enclosed by the inner coil and the leakage magnetic field outside the outer coil can be eliminated.
FIG. 1 shows an exemplary coil configuration for the outer shielding coil of the ASGC proposed by Roemer et al. in U.S. Pat. No. 4,737,716, which is designed to be used in conjunction with the gradient coil in a direction perpendicular to an axial direction of the static magnetic field. This coil configuration reflects the continuous distribution of the complete shielding eddy currents on a cylindrical shielding body surface along which the ASGC is to be provided.
In general, the eddy current distribution is expressed by a special function such as a modified Bessel function, so that it has been difficult to realize this distribution faithfully by arranging the physically discrete coils according to the conventionally available coil winding techniques, so that it has been inevitable to utilize a highly sophisticated manufacturing technique such as a numerical controlling (NC). In particular, in order to realize a higher shielding rate for the leakage magnetic field, a higher precision coil winding technique has been required, so that the manufacturing steps as well as the manufacturing cost required by the realization of the satisfactory ASGC could be quite enormous.
To this end, there has been a proposition for a considerable reduction of the manufacturing steps and the manufacturing cost required by the realization of the satisfactory ASGC by employing the etching manufacturing method for the formation of the ASGC winding pattern, as disclosed by Schenck et al. in U.S. Pat. No. 4,646,024. However, this reference completely fails to disclose any concrete teaching concerning the practical formation of the desired etching pattern.
For example, the etching manufacturing method is known to have a limit to the thickness of a metal plate such as a copper plate to which this method is applicable, so that there is a problem concerning the greater heat generation due to the increased resistance of the coil to which this method is applicable, compared to the coil materials available to the conventionally available coil winding techniques, but such a practical problem associated with the employment of the etching manufacturing method has not been addresses at all in this reference.
In addition, in the etching manufacturing method, the width of the conductive portion of the metal plate required for each turn of the coil winding becomes quite wide, so that there arises the problem of the self eddy currents appearing within the wide conductive portion of the metal plate for each turn of the coil winding, which can degrade the image quality considerably, but this practical important problem associated with the employment of the etching manufacturing method has also not been addresses at all in this reference.
Moreover, the ASGC is also associated with the problem that the ideal number of coil turns for the outer shielding coils must be approximated by the integer number of coil turns realizable in practice, so that only incomplete shielding property can be realizable and there is a residual eddy current magnetic field. This residual eddy current magnetic field has a negative sign so that it weakens the actual gradient magnetic fields in a case of under-shielding, whereas it has a positive sign so that it strengthens the actual gradient magnetic fields in a case of over-shielding.
Furthermore, the ASGC is also associated with the problem that the shielding property provided by the outer shielding coil functions to reduce the strength of the gradient magnetic fields generated by the inner gradient coil, so that it becomes necessary for the inner gradient coil to increase the number of turns in order to generate the gradient magnetic fields of the same strength with the same amount of the supplied gradient coil currents. As a result, the inductance and the resistance of the inner gradient coil as well as the work and the cost for manufacturing the inner gradient coil must be increased.
Now, various imaging pulse sequences to be used in the nuclear magnetic resonance imaging are also associated with the problem concerning an optimum setting of various system parameters for each imaging pulse sequence, as follows.
For example, in the one shot ultra high speed MRI pulse sequence, the major technical problem encountered is how to realize the reading pulse sequence which requires a large amplitude and high speed switching. Conventionally, this problem is resolved by using a multi-filer gradient coil scheme in which each gradient coil is divided into a plurality of sections to be driven in parallel by a plurality of linear amplifiers called multi-filers, such that the effective inductance of each gradient coil can be reduced to the normal inductance divided by a number of the multi-filers, so that the switching time can be reduced considerably.
However, there has been no known scheme for optimizing the system parameters such as the number of the multi-filers to be used, the spatial resolution to be achieved, and the data acquisition time to be realized. Thus, there has been no known prescription for minimizing the data acquisition time for a given number of the multi-filers and a given spatial resolution requirement. Similarly, there has been no known prescription for minimizing the number of the multi-filers for a given spatial resolution requirement and a desired data acquisition time. Likewise, there has been no known prescription for maximizing the spatial resolution for a given number of the multi-filers and a desired data acquisition time.
As a consequence, there has been no known method for optimum setting of the gradient coil configuration conditions suitable for optimizing these system parameters.
In addition, when the gradient coil configuration conditions specifying the basic structure of the gradient coils and the number of the multi-filers are given, there has been no known prescription for optimizing the other system parameters within these given configuration conditions. Thus, there has been no known prescription for minimizing the data acquisition time with respect to the given spatial resolution requirement under the given configuration conditions. Similarly, there has been no known prescription for maximizing the spatial resolution with respect to the desired data acquisition time under the given configuration conditions.
As a consequence, there has been no known method for optimum setting of the gradient coil operation condition suitable for optimizing these system parameters under the given configuration conditions.
Similar problems also arise for the divisional scanning ultra high speed MRI pulse sequence in which the spatial frequency bandwidth is widened by carrying out the data aquisition by a plurality of divided scans, in order to achieve the high level resolution unattainable by the one shot ultra high speed MRI.
Namely, there has been no known scheme for optimizing the system parameters such as the number of the divided scans to be carried out, the number of the multi-filers to be used, the spatial resolution to be achieved, and the data acquisition time to be realized. Consequently, there has been no known prescription for minimizing the data acquisition time for a given number of the divided scans, a given number of the multi-filers, and a given spatial resolution requirement. Similarly, there has been no known prescription for minimizing the number of the multi-filers for a given number of the divided scans, a given spatial resolution requirement, and a desired data acquisition time. Likewise, there has been no known prescription for maximizing the spatial resolution for a given number of the divided scans, a given number of the multi-filers, and a desired data acquisition time. Also, there has been no known prescription for minimizing the number of the divided scans for a given number of the multi-filers, a given spatial resolution requirement, and a desired data acquisition time.
As a consequence, there has been no known method for optimum setting of the gradient coil configuration conditions suitable for optimizing these system parameters.
In addition, when the gradient coil configuration conditions specifying the basic structure of the gradient coils and the number of the multi-filers are given, there has been no known prescription for optimizing the other system parameters within these given configuration conditions. Thus there has been no known prescription for minimizing the data acquisition time with respect to the given number of the divided scans and the given spatial resolution requirement under the given configuration conditions. Similarly, there has been no known prescription for maximizing the spatial resolution with respect to the given number of the divided scans and the desired data acquisition time under the given configuration conditions. Likewise, there has been no known prescription for minimizing the number of the divided scans with respect to the given spatial resolution requirement and the desired data acquisition time under the given configuration conditions.
As a consequence, there has been no known method for optimum setting of the gradient coil operation condition suitable for optimizing these system parameters under the given configuration conditions.
As for the nuclear magnetic resonance angiography (referred also as MRA hereinbelow) pulse sequence, the major technical problem encountered is how to realize the reading pulse sequence which requires a large amplitude and high speed switching, Just as in the case of the one shot ultra high speed MRI. Consequently, the multi-filer gradient coil scheme described above is also effective in this case.
However, there has been no known scheme for optimizing the system parameters such as the number of the multi-filers to be used, the reading direction spatial resolution to be achieved, and the each time to be realized. Thus, there has been no known prescription for minimizing the echo time for a given number of the multi-filers and a given reading direction spatial resolution requirement. Similarly, there has been no known prescription for minimizing the number of the multi-filers for a given reading direction spatial resolution requirement and a desired echo time. Likewise, there has been no known prescription for maximizing the reading direction spatial resolution for a given number of the multi-filers and a desired echo time.
As a consequence, there has been no known method for optimum setting of the gradient coil configuration conditions suitable for optimizing these system parameters.
In addition, when the gradient coil configuration conditions specifying the basic structure of the gradient coils and the number of the multi-filers are given, there has been no known prescription for optimizing the other system parameters within these given configuration conditions. Thus, there has been no known prescription for minimizing the echo time with respect to the given reading direction spatial resolution requirement under the given configuration conditions. Similarly, there has been no known prescription for maximizing the reading direction spatial resolution with respect to the desired echo time under the given configuration conditions.
As a consequence, there has been no known method for optimum setting of the gradient coil operation condition suitable for optimizing these system parameters under the given configuration conditions.
Similar problems also arise for the phase encoding pulse sequence to be used in the ultra high speed MRI and the MRA pulse sequences. Here, the number of the encoding steps is one in the one shot ultra high speed MRI or MRA, but the greater number of the encoding steps is usually required for the divided scanning ultra high speed MRI or MRA as a greater amount of the encoding is required. But, as the number of the encoding steps increases, the encoding pulse width becomes wider, so that the influence of the widened encoding pulse becomes significant for the switching time in the reading pulse sequence.
Yet, there has been no known scheme for optimizing the system parameters such as the number of the multi-filers to be used, the amount of encoding permitted, and the encoding pulse width necessary. Thus, there has been no known prescription for minimizing the encoding pulse width for a given number of the multi-filers, and a given amount of encoding. Similarly, there has been no known prescription for minimizing the number of the multi-filers for a given amount of encoding and a given encoding pulse width. Likewise, there has been no known prescription for maximizing the amount of encoding for a given number of the multi-filers and a given encoding pulse width.
As a consequence, there has been no known method for optimum setting of the gradient coil configuration conditions suitable for optimizing these system parameters.
In addition, when the gradient coil configuration conditions specifying the basic structure of the gradient coils and the number of the multi-filers are given, there has been no known prescription for optimizing the other system parameters within these given configuration conditions. Thus, there has been no known prescription for minimizing the encoding pulse width with respect to the given amount of encoding under the given configuration conditions. Similarly, there has been no known prescription for maximizing the amount of encoding with respect to the given encoding pulse width under the given configuration conditions.
Thus, there has been no known prescription for optimum setting of various gradient coil configuration conditions and the gradient coil operation condition suitable for optimizing various system parameters for each imaging pulse sequence, so that the operation efficiency realizable in the conventional nuclear magnetic resonance imaging has not been an optimum one.